A laboratory waste stream at contains ions

CHEMISTRY · Multi-part · AP · Chemistry · Acids and Bases · Acids and Bases

Problem

A laboratory waste stream at 25∘C25^\circ\text{C} contains 0.025 mol L−10.025\,\text{mol}\,\text{L}^{-1} Al3+Al^{3+} ions. Prior to discharge, the facility raises the pH so that Al(OH)3Al(OH)_3 will just begin to precipitate. Solubility-product data at 25∘C25^\circ\text{C}: Al(OH)3(s)↔Al3++3OH−Al(OH)_3 (s) \leftrightarrow Al^{3+} + 3OH^{-}; Ksp=3.0×10−10K_{sp} = 3.0 \times 10^{-10}. Assume activity coefficients are unity. (a) Calculate the hydroxide-ion concentration, [OH−][OH^{-}], at which precipitation of Al(OH)3Al(OH)_3 just begins in this waste stream. (b) Determine the pH corresponding to the threshold in part (a). Report your answer to two decimal places. (c) Briefly state why increasing the pH beyond the value found in part (b) results in additional precipitation of Al(OH)3Al(OH)_3.

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